Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e PA1 Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.fwdarw.H.sub.2 O PA1 (a) a first end plate; PA1 (b) a second end plate; PA1 (c) at least one fuel cell assembly interposed between the first and second end plates, the at least one fuel cell assembly comprising a membrane electrode assembly having a catalytically active region and further having at least one opening formed therein extending through the catalytically active region; PA1 (d) a compression assembly comprising at least one restraining member extending within the at least one opening, fastening means disposed at opposite ends of the at least one restraining member, and compressive means interposed between at least one of the fastening means and at least one of the first and second end plates.
In typical fuel cells, the MEA is disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field plate serves as an anode plate for one cell and the other side of the fluid flow field plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an exhaust manifold and outlet port for the coolant water exiting the stack.
In conventional fuel cell designs, such as, for example, the fuel cells described and illustrated in U.S. Pat. Nos. 3,134,697, 3,297,484, 3,297,490, 4,057,479, 4214,969 and 4,478,917, the end plates which make up each conventional fuel cell assembly are compressed and maintained in their assembled states by tie rods. The tie rods extend through holes formed in the peripheral edge portion of the end plates and have nuts or other fastening means assembling the tie rods to the fuel cell assembly and compressing the end plates of the fuel cell assembly toward each other. The reason for employing a peripheral location for the tie rods in conventional designs is to avoid the introduction of openings or otherwise interfering with the central, electrochemically active portion of the fuel cell.
In some conventional fuel cell stack assemblies, such as, for example, that described and illustrated in U.S. Pat. No. 5,176,966 (see FIG. 1), a hydraulic piston or bladder is installed adjacent one of the end plates. In such conventional arrangements, the hydraulic piston uniformly applies compressive force to the stack, permits control of the compressive force applied to the end plate, and allows for the expansion and contraction of the tie rods as they are heated and cooled during operation of the fuel cells.
In fuel cell stack assemblies in which hydraulic pistons are not employed, the use of springs in conjunction with tie rods is generally required to compress the stack and to maintain the compressive load over time. In general, the length of a fuel cell stack shortens over time due to the tendency of MEAs to gradually decrease in thickness while under compressive load. Optimally, the springs should impart a predetermined compressive load with minimal load variation over as large a deflection range as possible. When peripherally disposed tie rods are employed, each of the end plates securing the fluid flow field plates and MEAs must be greater in area (and therefore overhang) the fluid flow field plates and MEAs. The amount of overhang depends upon the diameter of the springs inserted at the ends of the tie rods between the end plates and the nuts securing the tie rods, since substantially all of the springs' diameter should be in contact with the end plate to provide effective and uniform compressive load.
In fuel cell stack applications, disc springs (sometimes referred to as Belleville washers) have been found to provide desirably high compressive load in a compact space. Additionally, disc springs are advantageous, particularly in comparison to other types of springs, in that, by selecting the proper thickness-to-diameter ratio, disc springs exhibit a flat load versus deflection curve at the upper half of their deflection range. This characteristic is most notably exhibited by disc springs having lower thicknesses. Thinner disc springs not only produce desirable load characteristics, but the overall height of a stack of disc springs is reduced, resulting in improved volumetric efficiency of the compression mechanism. However, for a given disc spring diameter, the compressive load produced by the spring decreases with decreasing material thickness. Therefore, in order to produce the same compressive load in a smaller volume, either the number of disc spring locations must be increased or the diameter of the springs must be increased.
The peripheral edge location of the tie rods in conventional fuel cell designs has inherent disadvantages. First, the peripheral location of the tie rods requires that the thickness of the end plates be substantial to provide the stiffness necessary to transmit the compressive force uniformly across the entire area of the end plate. End plates having insufficient thickness will deflect and will not adequately compress the central region of the various interior humidification and active section MEAs and fluid flow field plates interposed between the end plates. Inadequate compressive forces can compromise the seals associated with the manifold headers and flow fields in the central regions of the interior fluid flow field plates, and also compromise the electrical contact required along the surfaces of the fluid flow field plates to provide the serial electrical connection among the fuel cells which make up the stack. However, end plates of substantial thickness contribute significantly to the overall weight and volume of the fuel cell stack, both of which are preferable to minimize, particularly in motive fuel cell applications.
Additionally, the peripheral location of the tie rods requires that the area of the end plates be substantially greater than the area of the fluid flow field plates and MEAs in order to accommodate the springs interposed between the end plates and the nuts securing the tie rods. The increased area of the end plates to accommodate the springs increases the overall volume occupied by the fuel cell stack, thereby reducing its volumetric efficiency.
The peripheral edge location of fluid manifolds in conventional fuel cell stack designs is also disadvantageous. In order to provide sufficient structural strength to contain the elevated pressures within the manifolds, a significant thickness of constraining material must be provided between the interior of the manifolds and the outer edge of the flow field plate. This constraining material does not contribute to electrochemical activity, but does add to stack volume and weight.